1. Field of the Invention
This invention pertains to a novel flame retarding additive useful in polyolefin compositions, as well as the resulting flame retarded polyolefin compositions. The inventive flame retarding compositions are comprised of a mixture of two flame retardant additives, a halogenated bisphenol derivative, for example tetrabromobisphenol A bis(dibromopropyl ether), also referred to as "TBBPA-bis(DBP)", and the copolymer of a halogenated vinyl aromatic, for example bromostyrene, grafted onto certain polymers, such as polypropylene.
2. Description of the Prior Art
Polyolefins have desirable properties for a variety of applications. For example, polypropylene is a versatile light weight plastic characterized by low cost, an attractive glossy surface, and excellent solvent and stain resistance. Its attractive fundamental properties have spurred its adaptation to a long list of applications including films, fibers and molded articles. Its use as a fiber in home, commercial and institutional settings--as well as its role in molded parts for electrical applications--have brought about the need for improved flame resistance.
It is known that numerous bromine-based flame retarding agents can be added to polyolefins such as polypropylene to achieve desired ignition resistance. Examples of these additives are decabromodiphenyl oxide, hexabromocyclododecane, tetrabromobisphenol A, brominated polystyrene, bis(tribromophenoxy)ethane and the like. However, every one of the effective agents appears to impart one or more undesirable properties that result in compositions which are less attractive or acceptable than non-flame retardant Polypropylene. For example, most bromine-based flame retardants are fairly expensive and add signficantly to the cost of the propylene. Many bromine-based flame retardants are incompatible with polypropylene, and over time they exude to the surface, resulting in an oily or frosty appearance on molded parts. When spun into fibers, surface blooming flame retardants can be deposited onto textile handling equipment, building to the point that production must be stopped for cleaning.
Polypropylene is thermally stable at normal processing temperatures. However, the more efficient brominated flame retardants contain labile aliphatic halogen which tends to degrade under standard processing conditions, resulting in discoloration and corrosion. Also, most brominated flame retardants with sufficient thermal stability to withstand high temperature spinning conditions contain the more stable--but less efficient--aromatic bromine. Besides requiring higher loadings to compensate for lower efficiency, many of the thermally stable flame retardants have melting points above typical polypropylene spinning temperatures. Thus, while polypropylene is readily spun into fibers, this can cause abrasion and/or plugging of spinnerettes.
It is generally recognized in the industry that brominated flame retardants containing only aromatically bound bromine can lack efficiency in polyolefins, especially when the ignition source is a relatively cool flame. On the other hand, using flame retardants containing only the more loosely bound aliphatic bromine can result in decomposition during high temperature compounding. This will cause discoloration of the polymer and corrosion of the handling equipment.
TBBPA-bis(DBP) is an established commercial flame retardant known to be especially efficient in polypropylene. Numerous patents on its use exist, including U.S. Pat. No. 3,883,481 issued to Kopetz et al. on May 13, 1975, which describes a composition comprising a polyolefin, antimony oxide and a series of brominated phenoxy derivatives. TBBPA-bis(DBP), by itself, does have the fire retarding efficiency that was predicted and has acceptable thermal stability for many polypropylene applications. However, it is fairly expensive and also has a major fault that prevents even broader use. Like many other brominated flame retardants, it "blooms" or "bleeds" to the surface of polypropylene. This gives a frosty appearance to molded parts that is easily smeared, leaving an unattractive surface. In thin sections--and particularly in spun fibers--it may be possible to lose enough flame retardant to the surface that the fire retardancy is diminished.
The effectiveness of TBBPA-bis(DBP) has been so widely recognized that a significant amount of research has been done to find ways to eliminate its tendency to bloom. To date all of these attempts have centered on the addition of compatibilizers to retard the migration of TBBPA-bis(DBP) to the surface. However, deficiencies in these systems have persisted, and there has remained a desire for polyolefin compositions which include TBBPA-bis(DBP) and which have improved flame retarding efficiency but which show reduced or no bloom or bleed.